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Marine mammals, which include seals, whales, dolphins, otters and walruses, form a diverse group of 129 species that rely on the ocean for their existence.[1] They do not represent a distinct biological grouping, but rather are unified by their reliance on the marine environment for feeding.[2] The level of dependence on the marine environment for existence varies considerably with species. For example, dolphins and whales are completely dependent on the marine environment for all stages of their life, whereas seals feed in the ocean, but breed on land.[2]

Marine mammals can be subdivided into four recognised groups; cetaceans (whales, dolphins, and porpoises), pinnipeds (seals, sea lions and walruses), sirenians (manatees and dugongs), and fissipeds, which are the group of carnivores with separate digits (the polar bear, and two species of otter). Both cetaceans and sirenians are fully aquatic and therefore are obligate ocean dwellers. Pinnipeds are semiaquatic; they spend the majority of their time in the water, but need to return to land for important activities such as mating, breeding and molting. In contrast, both otters and the polar bear are much less adapted to ocean living.[2] While the number of marine mammals is small compared to those found on land, their total biomass is large. They play important roles in maintaining marine ecosystems, especially through regulation of prey populations.[3] These two factors make them an integral component of the marine environment. This is of particular concern considering 23% of marine mammal species are currently threatened.Taxonomy[edit]

A Polar bear (Ursus maritimus), a member of family Ursidae

A Sea Otter (Enhydra lutris), a member of family Mustelidae Mammals have returned to the water in at least nine separate evolutionary lineages (Cetacea, Sirenia, Desmostylia, Pinnipedia, Ursus maritimus (polar bear), Kolponomos (marine bear), Thalassocnus (aquatic sloth), Enhydra lutris (sea otter) and Lontra feline (marine otter)). Three of these lineages are extinct (Desmostylia; Kolponomos; Thalassocnus).[1] Despite the diversity in morphology seen between groups, improving foraging efficiency has been the main driver in the evolution in these lineages.[5] Today, marine mammals belong to one of three orders; Cetacea, Sirenia, or Carnivora. Within the Order Carnivora are the pinnipeds (sealions, walruses and seals), the polar bear (Ursus maritimus), and two otters (Enhydra lutris and Lontra feline).[2]Order Cetacea Suborder Mysticeti (baleen whales) Family Balaenidae (right and bowhead whales) = 2 genera; four species Family Neobalaenidae (pygmy right whale) = one species Family Balaenopteridae (rorquals) = 2 genera; 8 species Family Eschrichtiidae (gray whale) = 1 species Suborder Odontoceti (toothed whales) Family Physeteridae (sperm whale) = 1 species Family Kogiidae (pygmy and dwarf sperm whales = 1 genera; 2 species Family Monodontidae (narwhal and beluga) = two genera; two species Family Ziphiidae (beaked whales) = 6 genera; 21 species Family Delphinidae (oceanic dolphins) = 17 genera; 36 species Family Phocoenidae (porpoises) = 2 genera; 6 species Family Platanistidae (South Asia river dolphin) = 1 species Family Iniidae (boto) 1 species Family Lipotidae (Baji) probably extinct Family Pontoporiidae (franciscana) = 1 species Order Sirenia (sea cows) Family Trichechidae (manatees) = 1 genus; 3 species Family Dugongidae (dugongs) = 1 species Order Carnivora (carnivores): Family Mustelidae (otters) Enhydra lutris (sea otter) Lontra feline (marine otter) Family Ursidae (bears) Ursus maritimus (polar bear) Suborder Pinnipedia (sealions, walruses, seals) Family Otariidae (fur seals and sea lions) = 7 genera; 16 species Family Odobenidae (walrus) = 1 species Family Phocidae (true seals) = 13 genera; 18 species